The long term goals of this study are to examine two fundamental aspects of protein chemistry: 1) the mechanism of interaction of proteins and nucleic acids, particularly RNA; and 2) the structural basis of regulation of protein function by phosphorylation. In this proposal these goals will be addressed through characterization of a) the interactions of pp12, a type C retroviral nucleocapsid protein, with RNA, and b) the mechanism by which its RNA binding properties are modulated by phosphorylation. The specific aims are: 1) to characterize the interaction between pp12 and RNA by determining the size of the RNA binding site, the structural features of nucleic acids that determine the strength of the interaction, by identifying the amino acid residues in the RNA binding site and defining the atomic details of the interaction with nucleic acids; 2) to define the effect of dephosphorylation (to the p12 form) on the structural and functional properties of the pp12 protein determined in Aim 1; and 3) to evaluate the effect of single amino acid residues upon the structure of p12 and upon its interaction with RNA. Since pp12 is one of the smallest proteins (9.4 kd) for which phosphorylation is correlated with a change in conformation and in functional properties, it is an ideal system for elucidating the details of these changes. A variety of techniques including circular dichroism, multinuclear NMR, chemical modification, and site directed mutagenesis will be used to accomplish the aims.